FIDO U2F is a "Universal Second Factor" provided by FIDO Alliance, which works with several web services, like Facebook, Dashlane, Gmail, Dropbox, GitHub, etc.
For each of these web services, you need to set up the security parameters of your account to register your Nano S or your Blue as a second factor security key to authenticate on it. This second factor of verification will improve the security of your loging in processes, as you will be first required your login and password, second an authentication via your Nano S.
You don't need this FIDO application to manage your currencies, it is a privacy general feature.
TUTORIAL TO USE THE FIDO SECOND FACTOR OF VERIFICATION WITH NANO S OR BLUE
To let your Nano S or your Blue be recognized by the security parameters of services partnering with FIDO:
Then, to add your Ledger's device as second factor of authentication for: